2d graphics simulation of a ball in gravity field. Energy conservation law is respected.

physical_ball_2d_graphics.groovy

import javax.swing.*;
import java.awt.Color;
import java.awt.event.*;
import java.awt.geom.*;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;

public class Ball extends JComponent {

  public static final int physicsTimerInterval = 1; // milliseconds, means 1000 computes / second
  public static final int paintTimerInterval = 20; // milliseconds, means 50 frames / second

  private double scale = 50; // pixels/meter
  private double diameter = 1;
  private double x = diameter;
  private double y = diameter;
  private double vx = 3;
  private double vy = 0;
  private double g = 9.8;
  private double dt = physicsTimerInterval / 1000.0; // in seconds
  private double energy;
  private long stepsComputed = 0;

  public Ball() {
  }
  
  void computePhysics() {  
    Rectangle r = getBounds(null);
    double bx = r.x / scale;
    double by = r.y / scale;
    double bwidth = r.width / scale;
    double bheight = r.height / scale;
    if(stepsComputed == 0)
      energy = (vx*vx + vy*vy) / 2 + g * (bheight - y)
    if(x < bx + diameter / 2) {
      x = Math.max(x, bx + diameter / 2);
      vx = -vx;
    } else if (x > bx + bwidth - diameter / 2) {
      x = Math.min(x, bx + bwidth - diameter / 2);
      vx = -vx;
    }
    if(y < by + diameter / 2) {
      y = Math.max(y, by + diameter / 2);
      vy = -vy;
    } else if (y > by + bheight - diameter / 2) {
      y = Math.min(y, by + bheight - diameter / 2);
      vy = -vy;
    }
    vy += g * dt;
    x += vx * dt;
    // y += vy * dt;
    // respect energy conservation law
    y = bheight - (energy - (vx*vx + vy*vy) / 2) / g;
    ++stepsComputed;
  }

  public void paintComponent(Graphics g) {
    Graphics2D g2d = (Graphics2D)g;
    double screenX = x * scale;
    double screenY = y * scale;
    double screenDiameter = diameter * scale;
    Ellipse2D.Double circle = new Ellipse2D.Double(screenX - screenDiameter / 2, screenY - screenDiameter / 2, screenDiameter, screenDiameter);
    g2d.setPaint(new Color(0, 128, 128));
    g2d.fill(circle);  
  }
}

public class Main {
  private static void createAndShowGUI() {
    JFrame frame = new JFrame("Java 2d graphics test");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(800, 600);
    final Ball component = new Ball();
    frame.add(component);    
    final Timer physicsTimer = new Timer(Ball.physicsTimerInterval, new ActionListener() {
      public void actionPerformed(ActionEvent evt) {
        component.computePhysics();
      }
    });
    final Timer repaintTimer = new Timer(Ball.paintTimerInterval, new ActionListener() {
      public void actionPerformed(ActionEvent evt) {
        component.repaint();
      }
    });
    physicsTimer.start();
    repaintTimer.start();
    frame.addWindowListener(new WindowAdapter() {
      public void windowClosing(WindowEvent e) {
        physicsTimer.stop();
      }
    });
    frame.setVisible(true);
  }

  public static void main(String[] args) {
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
      public void run() {
        createAndShowGUI();
      }
    });
  }
}

new Main().main([] as String[])